This invention relates to the measurement of electrical impedance, and in particular to the measurement of the loss and the very precise measurement of the capacitance of an unknown impedance where "loss" is used as a collective term to mean resistance, conductance, dissipation factor or any other term used to describe the real component of impedance.
The technical literature is replete with numerous examples of impedance bridges of all kinds. Bridges have been in a state of continuous development and improvement for more than a century. Improvements have taken almost every conceivable form, in efforts to achieve higher accuracy, lower cost, better reliability, higher speed, wider range, etc. More recently, most high performance bridges have been automated with the incorporation of microprocessors or related devices to allow these bridges not only to correct for various measurement errors, but to report their measurement results on sophisticated local displays or remotely via several different kinds of communication channels. Sufficient programming control is often provided to allow for sustained unattended operation.
In spite of the considerable attention given to impedance bridges in general, not all areas of bridge development have benefited from new ideas, particularly in the application of microprocessors. One such area is the construction of ratio transformer bridges for high precision measurements of capacitance and loss. Commercially, the state of the art is represented by the GenRad (formerly the General Radio Co.) Model 1615A Capacitance bridge. A similar, slightly higher precision capacitance bridge was the GenRad Model 1616. Other similar ratio transformer bridges have been made in the past by companies such as Electro Scientific Industries and Wayne Kerr. Although these bridges have accuracies as high as 0.001%, they are all antiquated by today's standards, requiring manual operation by a skilled operator, having large numbers of manually operated switches which are prone to wear and thus reliability problems, and many sources of error which can only be corrected for with some effort on the part of the operator. Obviously, the speed with which such bridges can be operated is very slow since the operator must both balance the bridge and record the measurement, a process that takes an experienced person at least a minute.
More recently, quite a number of highly automated impedance bridges have become available such as the Hewlett-Packard 4274, the GenRad 1689, the Electro Scientific Industries 5100, the Boonton 76A and the Wayne Kerr 905. While these companies make some very flexible, fast, easy to use bridges capable of measuring a wide range of parameters, the very best is rated for measuring capacitance to an accuracy of only 0.02% under ideal conditions. While these products have found many uses, their limited accuracy prevents their application in situations where only the accuracy and resolution provided by a ratio transformer bridge such as the GenRad 1615A is adequate.
One manufacturer, Tettex, does make several automatic capacitance bridges which incorporate ratio transformers of which the Model 2876 is the most advanced. However, unlike the Model 1615A, these bridges use transformers where a ratio of currents is used to balance the bridge rather than a ratio of voltages as in the Model 1615A. The Model 2876 also uses a single external capacitance standard rather than multiple internal capacitance and resistance standards like the Model 1615A. The Tettex bridges are somewhat specialized in that they are designed to operate at very high voltages but are only accurate to 0.05% at best, and thus are not quite as good in this respect as the automatic impedance bridges described above.
The technical literature currently contains very little regarding specific implementations of automatic high precision capacitance bridges. An exception to this is a recent article by Robert D. Cutkosky, "An Automatic High-Precision Audiofrequency Capacitance Bridge", IEEE Transactions on Instrumentation and Measurement, Vol. IM-34, No. 3, September 1985. The design of a modern capacitance bridge is discussed using conventional high precision techniques combined with modern digital circuitry. The construction of this bridge is quite different from and more conventional than the construction of the present invention.